Polyimide fibers as a kind of high-performance fibers have many outstanding properties, such as high strength, high modulus, resistance to both high and low temperatures, and to radiation, flame and chemical corrosion, and desirable biocompatibility and dielectric properties. They have been used in a broad range of applications, including atomic energy industry, space environment, rescue, aeronautics and astronautics, national defense, new-type buildings, high-speed transport means, ocean development, sports equipment, new energies, environmental industry and protective appliances.
Currently, there are two commonly used methods to prepare polyimide fibers. One is a one-step process, in which a polyimide solution is used as a spinning solution, and polyimide fibers are spun by either a wet or a dry-wet process from the spinning solution. After preliminary stretching, the fibers possess certain strength. After removal of solvent, thermal stretching and thermal treatment (300° C.˜500° C.) are conducted to obtain high-strength, high-modulus polyimide fibers. Although this method features a simple spinning process, the commonly used solvents in synthesizing polyimide are phenol-based (e.g., cresol and parachlorophenol) which are highly toxic and have high boiling points, as a result, the residual solvent in fibers could not be completely removed. Therefore, this method is not environmentally friendly and is not suitable for industrial application. The Chinese invention patent ZL 02112048.X and the U.S. patents U.S. Pat. No. 4,370,290 and U.S. Pat. No. 5,378,420 all disclose such one-step polyimide fiber preparation method. Another method is the one using a two-step process, in which polyamic acid fibers are first obtained by spinning a concentrated polyamic acid solution using either a wet or a dry-wet process. The polyamic acid fibers as prepared are then chemically or thermally cyclized and stretched to obtain polyimide fibers. For example, the Japanese patents JP3287815 and JP4018115 both adopt this method to prepare polyimide fibers. The raw materials (diamines and dianhydrides) and polyamic acid precursor polymers are soluble in a number of solvents having lower toxicity and boiling points. Thus, the two-step method can overcome the processing difficulties resulting from the infusibility and insolubility of polyimide fibers. In addition, the amounts of residual solvent in the fibers are low. However, the properties of the polyamic acid fibers obtained in the initial polymerization step will deteriorate over time, and after winding of polyamic acid fibers, the small amount of residual solvents that are still left in the fibers will become nonvolatile and will affect the storage and performance of polyamic acid fibers, therefore the performance of the polyimide fibers converted from the polyamic acid fibers is not very high. Moreover, the multiple steps adopted in this method make it unsuitable for continuous production. Accordingly, there remains a need to develop new processing methods for making high quality polyimide fibers suitable for continuous industrial production.